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Enums

In Mint enums represents Algebraic Data Types, with them it's possible to describe data which contains different types of values (called variants).

For example a type for a logged in state can be written as two variants:

enum UserState {
  LoggedIn(User)
  Visitor
}

since this is a type it can be used in type signatures:

fun isLoggedIn (userState : UserState) : Bool {
  case (userState) {
    UserState::LoggedIn user => true
    UserState::Visitor => false
  }
}

isLoggedIn(UserState::LoggedIn(user)) /* true */
isLoggedIn(UserState::Visitor) /* false */

as you can see from the code above you can create instances of the type by using it's name then a double colon then it's variant and then any arguments it takes UserState::LoggedIn(user) , also you can match the variants in a case expression.

Type variables

You can define type variables for an enum so it can become generic meaning that a type of a value of a variant can be any other type.

The best example for this is the Result(error, value) type:

enum Result(error, value) {
  Err(error)
  Ok(value)
}

which can be used with any types for error and value:

/* A result where the error and value is both string */
Result(String, String)

/* An example result type for HTTP requests. */
Result(Http.ErrorResponse, Response)